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United States Patent |
5,582,954
|
Swatton
,   et al.
|
December 10, 1996
|
Process for preparing photohardenable elastomeric element having
increased exposure latitude
Abstract
A process for preparing a photohardenable elastomeric element having
increased exposure latitude, said element having a support and a
photohardenable layer wherein at least (a) a thermoplastic-elastomeric
polymeric binder comprising a copolymer of isoprene, (b) a liquid
plasticizer, and (c) a petroleum wax, are premixed is described.
Inventors:
|
Swatton; David W. (N. Middletown, NJ);
Feinberg; Bernard (Englishtown, NJ);
Weikart; Udo (Obernburg, DE)
|
Assignee:
|
E.I. Du Pont de Nemours and Company (Wilmington, DE)
|
Appl. No.:
|
458631 |
Filed:
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June 2, 1995 |
Current U.S. Class: |
430/281.1; 430/270.1; 430/280.1 |
Intern'l Class: |
G03C 001/73 |
Field of Search: |
430/281,270,280
|
References Cited
U.S. Patent Documents
3989609 | Nov., 1976 | Brack | 522/33.
|
4218294 | Aug., 1980 | Brack | 522/33.
|
4517278 | May., 1985 | Sakurai | 430/286.
|
4686172 | Aug., 1987 | Worns et al. | 430/286.
|
Foreign Patent Documents |
4423358 | Aug., 1991 | EP.
| |
Primary Examiner: Lesmes; George F.
Assistant Examiner: Weiner; Laura
Parent Case Text
This is a continuation of application Ser. No. 08/224,051 filed Apr. 7,
1994, now abandoned, which is a continuation of application Ser. No.
07/829,998 filed Jan. 31, 1992, now abandoned.
Claims
We claim:
1. A process for preparing a photohardenable elastomeric element having a
support and a photohardenable layer, said photohardenable layer comprising
(a) a thermoplastic-elastomeric polymeric binder comprising a copolymer of
isoprene, (b) a liquid plasticizer, (c) a petroleum wax, (d) a
photoinitiating system and (e) an addition polymerizable nongaseous
ethylenically unsaturated monomer, the process comprising:
(A) premixing the binder, the plasticizer and the wax to form a homogeneous
first mixture;
(B) forming the first mixture into pellets by (i) feeding the first mixture
into a single-screw extruder, (ii) extruding the first mixture through a
die plate, (iii) chopping the extruded mixture to obtain pellets, and then
(iv) applying a stream of water to the pellets;
(C) heating the pellets at a temperature in the range of 130.degree. to
200.degree. C. with mixing in an extruder and with the addition of the
photoinitiating system and the monomer to form a second mixture; and
(D) forming the second mixture into a sheet structure onto a support.
2. A process for preparing a photohardenable elastomeric element having a
support and a photohardenable layer, said photohardenable layer comprising
(a) a thermoplastic-elastomeric polymeric binder comprising a copolymer of
isoprene, (b) a liquid plasticizer, (c) a petroleum wax, (d) a
photoinitiating system and (e) an addition polymerizable nongaseous
ethylenically unsaturated monomer, the process comprising:
(A) premixing the binder, the plasticizer, the wax and the photoinitiating
system to form a homogeneous first mixture;
(B) forming the first mixture into pellets by (i) feeding the first mixture
into a single-screw extruder, (ii) extruding the first mixture through a
die plate, (iii) chopping the extruded mixture to obtain pellets, and then
(iv) applying a stream of water to the pellets;
(C) heating the pellets at a temperature in the range of 130.degree. to
200.degree. C. with mixing in an extruder and with the addition of the
monomer to form a second mixture; and
(D) forming the second mixture into a sheet structure onto a support.
3. A process for preparing a photohardenable elastomeric element having a
support and a photohardenable layer, said photohardenable layer comprising
(a) a thermoplastic-elastomeric polymeric binder comprising a copolymer of
isoprene, (b) a liquid plasticizer, (c) a petroleum wax, (d) a
photoinitiating system and (e) an addition polymerizable nongaseous
ethylenically unsaturated monomer, the process comprising:
(A) premixing the binder, the plasticizer, the wax and the monomer to form
a homogeneous first mixture;
(B) forming the first mixture into pellets by (i) feeding the first mixture
into a single-screw extruder, (ii) extruding the first mixture through a
die plate, (iii) chopping the extruded mixture to obtain pellets, and then
(iv) applying a stream of water to the pellets;
(C) heating the pellets at a temperature in the range of 130.degree. to
200.degree. C. with mixing in an extruder and with the addition of the
photoinitiating system to form a second mixture; and
(D) forming the second mixture into a sheet structure onto a support.
4. A process for preparing a photohardenable elastomeric element having a
support and a photohardenable layer, said photohardenable layer comprising
(a) a thermoplastic-elastomeric polymeric binder comprising a copolymer of
isoprene, (b) a liquid plasticizer, (c) a petroleum was, (d) a
photoinitiating system and (e) an addition polymerizable nongaseous
ethylenically unsaturated monomer, the process comprising:
(A) premixing the binder, the plasticizer, the wax, the photoinitiating
system and the monomer to form a homogeneous first mixture;
(B) forming the first mixture into pellets by (i) feeding the first mixture
into a single-screw extruder, (ii) extruding the first mixture through a
die plate, (iii) chopping the extruded mixture to obtain pellets, and then
(iv) applying a stream of water to the pellets;
(C) heating the pellets at a temperature in the range of 130.degree. to
200.degree. C. with mixing in an extruder to form a second mixture; and
(D) forming the second mixture into a sheet structure onto a support.
5. The process of claims 1, 2, 3, or 4 wherein the premixing in step (A) is
carried out for about 2 to about 30 minutes to obtain an homogeneous
mixture.
6. The process of claims 1, 2, 3, or 4 wherein the binder is
styrene-isoprene-styrene.
7. The process of claims 1, 2, 3, or 4 wherein the plasticizer consists
essentially of liquid polyisoprene.
8. The process of claims 1, 2, 3, or 4 wherein an aromatic resin is added
to the first mixture formed in step (A) and is present in an amount of 1
to 15% by weight of the total photohardenable layer.
9. The process of claim 8 wherein the resin is a copolymer of
alpha-methylstyrene and vinyltoluene.
10. The process of claims 1, 2, 3, or 4 wherein the stream of water
contains a dispersing agent.
11. The process of claim 10 wherein the dispersing agent is calcium
stearate.
12. The process of claims 1, 2, 3, or 4 wherein the pellets are formed into
a sheet structure on a support by forming said pellets into a hot melt at
a temperature of about 130.degree. to 200.degree. and then extruding and
calendering the hot melt between a support and a multilayer-cover element.
13. The process of claims 1, 2, 3, or 4 wherein step (A) is carried out in
a mixing device selected from the group consisting of plough blade mixers,
ribbon mixers, paddle mixers, tumbling mixers, gravity mixers and Banbury
mixers and step (B) is carried out in an extruder.
14. The process of claims 1, 2, 3, or 4 wherein step (A) is carried out at
a temperature in the range of 100.degree. to 170.degree. C.
15. The process of claims 1, 2, 3, or 4 wherein the die plate has holes
about 0.25 to 0.5 cm in diameter and the pellets have a size in the range
of about 0.3 to about 1.0 cm in diameter.
Description
FIELD OF THE INVENTION
This invention relates to a process for preparing a photohardenable
elastomeric element which can be used to make a flexographic printing
plate. More particularly, this invention relates to an improved process
for preparing a photohardenable elastomeric element having an increased
exposure latitude.
BACKGROUND OF THE INVENTION
Photohardenable elements have a support and a layer of photosensitive
material, which is photopolymerizable or photocrosslinkable, applied
thereon. Photohardenable elements can be used to prepare flexographic
printing plates. Such elements generally comprise (1) an addition
polymerizable nongaseous ethylenically unsaturated monomer, (2) a
photoinitiating system activated by actinic radiation, and (3) an
elastomeric or thermoplastic-elastomeric polymeric binder comprising
polymerized conjugated diene monomers. Photohardenable compositions such
as these frequently contain plasticizers to increase the softness and
flexibility of the developed flexographic plates.
Processes for making photohardenable elements and producing flexographic
printing plates therefrom are well known in the art. Photohardenable
elements are typically prepared by mixing the monomer, the photoinitiator,
and the binder, usually at an elevated temperature. A plasticizer is then
added to the mixture and the resulting mixture is then formed into a sheet
structure by several known methods such as solvent casting, hot pressing,
calendering and extrusion. Generally, the mixture is formed into a sheet
structure by using an extrusion and calendering process. Once the
photopolymer composition is in the form of a sheet, the element can then
be used to make a printing plate by first exposing the element imagewise
to actinic radiation. The areas of the photohardenable layer which are
exposed to the radiation are photopolymerized or photocrosslinked and
thereby become less soluble in developer solvents. The unexposed or
unpolymerized areas of the element are subsequently washed off using a
suitable developer solvent. There can be additional post-development
treatments such as detackification and post-exposure.
There are problems associated with making a photohardenable element using
the above-described process. For example, it is often difficult to add
plasticizer materials to the other components of the photopolymerizable
composition if the plasticizer used is a highly viscous material. The
plasticizer cannot be added directly to the other components prior to
mixing because it causes agglomeration of the solid materials. Similarly,
when the plasticizer is added after mixing has begun, as in a later stage
of an extruder, it is difficult to pump the plasticizer from its drum into
the mixing unit. Residual plasticizer remaining in the drum also presents
a disposal problem. Thus, for environmental reasons, as well as ease of
operation, an alternative process is needed.
European patent application EP 0 442 358, published Aug. 21, 1991 describes
a process in which the liquid constituents of the photohardenable mixture
are premixed with the elastomeric binder, which is granular, until they
are absorbed by the binder. The mixture is then dusted with a powder which
is not soluble in the liquids. This process is disadvantageous because an
additional powdered material is introduced into the photohardenable
composition. Such powdered material adds dust, is difficult to handle, and
may adversely impact the properties of the composition.
There are also problems associated with the process to prepare the
flexographic printing plate from the photohardenable element. For example,
when exposing the element imagewise to actinic radiation, it is frequently
difficult to obtain the proper exposure time. The exposure time must be
long enough to polymerize small areas, i.e., the highlight dots, so that
these areas are not washed away with the developer solvent. At the same
time, the exposure time must not be too long or fine lines, which are
intended to remain unexposed and be washed out, i.e., the reverse lines,
will polymerize and not wash out. In either case, if the exposure is too
short or too long, the resolution and detail of the final plate is
inadequate. It is particularly difficult to obtain the proper exposure
with plates in which the elastomeric binder is a copolymer of isoprene,
e.g., a styrene-isoprene-styrene block copolymer. It has been found that
the photoactivity is greater and the exposure latitude even lower in
flexographic plates containing these binders.
It is therefore desirable to have a process for preparing photohardenable
elastomeric elements and flexographic printing plates which overcomes the
above-described disadvantages.
SUMMARY OF THE INVENTION
The present invention provides a process for preparing a photohardenable
elastomeric element having a support and a photohardenable layer which
comprises:
(A) premixing at least (a) a thermoplastic-elastomeric polymeric binder
comprising a copolymer of isoprene, (b) a liquid plasticizer and (c) a
petroleum wax to form a first mixture;
(B) optionally mixing (d) a photoinitiating system activated by actinic
radiation and/or (e) an addition polymerizable nongaseous ethylenically
unsaturated monomer, to form a second mixture;
(C) mixing the first and second mixtures to form a photohardenable mixture;
and
(D) forming the photohardenable mixture into a sheet structure onto a
support;
wherein if either the (e) monomer or (d) photoinitiator is not added in
step (B), it is added in Step (A); and
wherein if the (d) photoinitiator and (e) monomer are not added in step (B)
they both are added in Step (A).
In a preferred embodiment of this invention, the first mixture, i.e. the
premixture, is formed into pellets after step (A). The pellets are then
mixed with any optional remaining components, such as the monomer and/or
photoinitiator, to obtain a photohardenable mixture which is then formed
into a sheet structure by an extrusion process and further calendered
between the support and a multilayer cover element. The multilayer cover
element consists essentially of a flexible cover film, a layer of an
elastomeric composition and, optionally, a flexible polymeric film. The
elastomeric layer comprises an elastomeric polymeric binder, a second
polymeric binder and, optionally, a nonmigratory dye or pigment.
DETAILED DESCRIPTION OF THE INVENTION
Surprisingly and unexpectedly, it has been found that a photohardenable
elastomeric element having increased exposure latitude can be obtained by
(a) premixing at least a liquid plasticizer, a thermoplastic-elastomeric
polymeric binder and a petroleum wax to form a homogeneous premixture; (b)
mixing the premixture a second time to form a photohardenable mixture and
(c) forming the photohardenable mixture into a sheet structure on a
support.
The first step in the process of the invention is the premixing of at least
three components until a homogeneous mixture is obtained. By the term
"premixing" it is meant that the components are mixed a first time to form
a homogeneous mixture. This premixing step is in addition to the mixing
that takes place prior to the step in which the photohardenable mixture is
formed into a sheet structure. The three components which are premixed are
the binder, the plasticizer and the petroleum wax.
The premixing of the binder, plasticizer and petroleum wax, can take place
in any conventional mixing apparatus, including plough blade mixers,
ribbon mixers, paddle mixers, tumbling mixers and gravity mixers. A
Banbury mixer or similar mixing device is particularly suitable. The
premixing is carried out at a temperature slightly above room temperature,
generally in the range of 50.degree. to 200.degree. C. depending on the
nature of the materials. Preferably the temperature is in the range of
100.degree. to 170.degree. C. The binder, plasticizer, and wax are
premixed for a time sufficient to provide an approximately homogeneous
mixture. Usually a time of about 2 to 30 minutes is sufficient.
The binder used in the process of the invention is an elastomeric polymeric
material comprising polymerized isoprene monomers. A preferred binder is a
copolymer of styrene and isoprene, particularly a block copolymer of the
A-B-A type, i.e., styrene-isoprene-styrene. Such binders are discussed in,
e.g., Chen, U.S. Pat. No. 4,323,636. The binder used in the process of the
invention is generally present in an amount of 50 to 90% by weight of the
total photohardenable layer. It is preferred that the binder be present in
at least an amount of 65% by weight of the total photohardenable layer.
The plasticizer used in the process of the invention is a material which is
compatible with the binder and which increases the softness and
flexibility of the photohardenable layer and resulting flexographic plate.
The plasticizer must be compatible with the binder to the extent that a
clear, non-cloudy photohardenable layer is produced. Examples of suitable
plasticizers include process oils including aromatic, naphthenic and
paraffinic compounds, e.g., the Shellflex materials from Shell Chemical
Co. (Houston, Tex.), the Tufflo materials from Atlantic Richfield Co; the
Polypropene materials from Amoco Chemical Corp.; and low molecular weight
diene polymers such as butadiene or isoprene polymers, e.g., the
Polybutene materials from Chevron Chemical Co. and Polyoil from Nudex,
Inc. (Piscataway, N.J.). A preferred plasticizer is liquid polyisoprene;
particularly preferred is polyisoprene having a molecular weight in the
range of 25,000 to 50,000.
The plasticizer should be present in an amount which is sufficient to
impart softness and flexibility to the developed printing relief, without
causing a deleterious effect on the other properties of the
photohardenable element. Generally, the amount of plasticizer is in the
range of about 1 to 30%, preferably 5 to 15%, based on the weight of the
photohardenable layer.
The petroleum wax is added to prevent the premixture from being excessively
sticky or breaking up into agglomerates. This is particularly important
when the premixture is to be formed into small units, such as pellets.
Suitable waxes are tasteless and odorless solids consisting of a mixture
of solid hydrocarbons. Both paraffin waxes and microcrystalline waxes can
be used. Microcrystalline waxes are preferred as they tend to form more
physically stable mixes. A particularly preferred wax is Ceresin wax. The
petroleum wax is generally present in an amount of about 0.5 to 5.0%,
based on the weight of the photohardenable layer.
It is preferred that the premixture also contain an aromatic resin. The
aromatic resin is generally added to improve the extrudability of the
photohardenable layer. When the binder is a block copolymer of isoprene
and an aromatic hydrocarbon, e.g., styrene-isoprene-styrene, the aromatic
resin is compatible predominantly with the aromatic segment. The aromatic
resin is generally a polymeric material having a high solubility parameter
and a softening point above about 85.degree. C. Examples of suitable
resins include alkylated aromatic polyindene resins, coumarone-indene
resins, low molecular weight polystyrene, polymers and copolymers of
alpha-methylstyrene. A preferred resin is a copolymer of
alpha-methylstyrene and vinyltoluene. The aromatic resin is generally
present in an amount of 1 to 15% by weight of the total photohardenable
layer.
Although not preferred, it is possible to have the premixture contain all
of the components in the photohardenable composition. In this embodiment,
all of the components in the photohardenable composition, i.e., the
binder, the plasticizer, the petroleum wax, the monomer, the
photoinitiator and suitable additives, are premixed in a first mixing step
and then mixed again prior to formation into a sheet structure. The
additional components in the photohardenable composition, i.e., the
monomer, photoinitiating system, and additives, will be discussed in
greater detail below.
In a preferred embodiment, the premixture is divided into small units prior
to the second mixing step. This is done mainly to aid in the handling of
the premixture. The small units can be in the shape of a sphere, cube,
pellet, string, or filament, etc, or they can be irregular in shape. The
formation of the premixture into small units can be accomplished, for
example, by chopping the premixture into pieces, or by placing the
premixture into an extruder and extruding the mixture through a die plate
with the appropriately shaped openings. The small units can be of almost
any size which facilitates handling.
In a particularly preferred embodiment, the premixture is formed into
pellets having a size in the range of about 0.3 to about 0.8 cm in
diameter. It is preferred that this be accomplished by feeding the molten
premixture into a single-screw extruder and extruding the mixture through
a die plate with holes approximately 0.25 to 0.5 cm in diameter. The
extruded material is then chopped into pellets approximately 0.3 to 1.0 cm
long by a rotating knife on the die plate. The pellets are then carried
away by a stream of water. The water can contain a small amount, about 0.1
to 10% by weight, of a dispersing agent to prevent agglomeration of the
pellets. Suitable dispersing agents include quaternary ammonium compounds,
sulfonated oils, polyhydric alcohol ester and ethers, and salts or esters
of acids such as oleic, lauric or stearic acid. A preferred dispersing
agent is calcium stearate.
The next step is to mix again the premixture, as formed or divided into
small units. It is at this point that all the remaining components of the
photohardenable composition are added. It is preferred that at least the
monomer be added separately to the premixture in this step. It is
preferred that the mixture be made by forming the composition into a hot
melt. This can be accomplished by placing the components in a mixing
device such as a rubber mill, which may be part of a calender device. The
melting and mixing can also be carried out in the first stages of the
extruder which also performs the functions of deaerating and filtering the
composition. Although any order of addition can be used, it is preferred
to first place the premixture in the mixing device or extruder. The
monomer, initiator or other additives are subsequently added to the
premixture. A suitable extruder is a twin screw extruder, although other
known commercial extruders can be used. The temperature of the melt in the
extruder is within the range of about 130.degree. to 200.degree. C., and
the composition remains in the extruder from about 1 to 5 minutes.
Monomers which are suitable in the photohardenable composition are
addition-polymerizable ethylenically unsaturated compounds. The
photohardenable material can contain a single monomer or a mixture of
monomers which must be compatible with the binder to the extent that a
clear, non-cloudy photohardenable layer is produced. Monomers that can be
used are well know in the art. Examples of such monomers can be found in
Chen, U.S. Pat. No. 4,323,636; Fryd et al., U.S. Pat. No. 4,753,865; Fryd
et al., U.S. Pat. No. 4,726,877 and Feinberg et al., U.S. Pat. No.
4,894,315. Preferred monomers include the acrylate and methacrylate mono-
and poly-esters of alcohols and polyols such as alkyl alcohols,
hexamethylene glycol, trimethylol propane, polyoxypropyltrimethylol
propane, pentaerythritol, dipentaerythritol and the like. It is preferred
that the monomer be present in at least an amount of 5% by weight of the
photohardenable layer.
The photoinitiating system can be any organic compound or group of
compounds which is radiation sensitive, free-radical generating, and which
initiates polymerization of the monomer or monomers without excessive
termination. It should be activatable by actinic radiation and thermally
inactive at and below 185.degree. C. Photoinitiating systems of this type
include the substituted and unsubstituted polynuclear quinones, aromatic
ketones, benzoin and benzoin ethers, and 2,4,5-triarylimidazolyl dimers.
Examples of suitable systems have been disclosed in Gruetzmacher et al.,
U.S. Pat. No. 4,460,675 and Feinberg et al., U.S. Pat. No. 4,894,315.
Initiator systems are present in amounts from about 0.001% to 10% based on
the weight of the photohardenable composition.
The photohardenable composition can also contain other additives depending
on the final properties desired. Such additives include thermal
polymerization inhibitors, antioxidants, antiozonants, colorants, fillers
or reinforcing agents. These can be added to the components which are
combined to form the premixture in step (A), or they can be added when the
premixture is mixed to form the photohardenable mixture in steps (B) and
(C). In general, most fillers and reinforcing agents will be added to the
first components. All other ingredients will be added in step (B).
The final step in the process of the invention is the formation of the
above photohardenable material, i.e., the mixture of the premixture and
the optional remaining components, into a sheet structure on a support.
The photohardenable material can be formed into sheets or layers by
several known methods such as solvent casting, hot pressing, calendering
and extrusion. It is preferred that the photohardenable material be
extruded through a die slot directly onto the support and treated to
achieve the desired thickness. It is preferred that this be accomplished
by calendering the photohardenable material between two sheets or films
such as the support and a cover layer. Alternatively, the calendering of
the photohardenable material can occur between two temporary supports.
Calendering is accomplished by passing the photohardenable material from
the mixing device or extruder into the nip of a calender and calendering
while hot between the support and a cover layer.
Alternatively, the photohardenable material can be placed between the
support and a cover layer in a mold. The material is then pressed flat by
the application of heat and/or pressure.
Materials suitable for the support include metals, e.g., steel and aluminum
plates, sheets and foils, films or plates composed of various film-forming
synthetic resins or polymers, such as the addition polymers and linear
condensation polymers. Preferred support materials are polyester films,
such as polyethylene terephthalate.
A transparent cover sheet such as a thin film of polystyrene, polyethylene,
polypropylene or other strippable material can be used as a cover element
to prevent contamination of or damage to the photohardenable layer during
storage or manipulation. The cover sheet can also include a thin, hard,
flexible solvent-soluble layer, e.g., a polyamide layer, or a copolymer of
polyethylene and polyvinyl acetate, etc. This can be placed on the upper
surface of the photohardenable layer to protect for reuse the
image-bearing negative or transparency superposed thereon, or to improve
contact or alignment with the photohardenable surface.
In practicing the process of the invention, it is preferred to calender the
photohardenable material, between a support and a multilayer cover element
wherein said multilayer cover element consists essentially of a flexible
cover film, optionally a flexible polymeric film, and a layer of an
elastomeric composition which is photohardenable or becomes
photohardenable by contact with the photohardenable layer. Such multilayer
cover elements are disclosed in Gruetzmacher et al., U.S. Pat. Nos.
4,427,759 and 4,460,675. The flexible cover film, which can be subcoated,
is generally a polyester, polystyrene, polyethylene, polypropylene or
other strippable film. The optional flexible polymeric film layer is
present adjacent to the cover film. This layer is soluble or strippable in
developer solutions during processing of the exposed element, and can have
the same composition as the solvent-soluble layer discussed above. The
optional flexible polymeric film is preferably a polyamide or a copolymer
of ethylene and vinyl acetate.
The elastomeric layer of the multilayer cover element should have an
elastic modulus in the polymerized state not substantially less than the
elastic modulus of the photohardenable layer in the exposed state. The
elastomeric composition comprises an elastomeric polymeric binder, a
second polymeric binder and optionally a nonmigratory dye or pigment. The
elastomeric composition can also contain a monomer or monomers and a
photoinitiating system. The elastomeric polymeric binder in the
elastomeric composition is generally the same as or similar to the
elastomeric binder present in the photohardenable layer.
In general, the process of preparing a flexographic printing plate from a
photohardenable element includes the steps of (1) main imagewise exposure,
(2) development or washout, and (3) post-development treatment.
Post-development treatment can include any or all of the following:
drying, overall post-exposure and surface detackification. A backflash
exposure may also be used with elements having a transparent or
translucent support. The backflash exposure step is carried out before the
main imagewise exposure step and gives the photohardenable layer a uniform
and relatively short exposure through the support, thereby
photocrosslinking the binder and monomer in the support region, i.e.,
creating a "floor". This serves to sensitize the plate and establishes the
depth of the plate relief. The backflash exposure step generally uses a
radiation source the same as or similar to that used for the main
imagewise exposure, as discussed below.
Printing reliefs can be made from the photohardenable elements described
above, by exposing to actinic radiation selected portions of the
photohardenable layer through an image-bearing transparency. The
image-bearing transparency can be constructed of any suitable material
including cellulose acetate film and oriented polyester film. Upon
exposure to actinic radiation, the ethylenically unsaturated monomer is
polymerized or crosslinked in those areas of the photohardenable layer
exposed to actinic radiation resulting in reduced solubility or
swellability in developer solvents. No significant polymerization or
crosslinking occurs in the unexposed areas of the photohardenable layer.
Consequently, upon application of a suitable developer solvent, the
exposed areas remain and the unexposed areas are removed from the
photohardenable layer.
Actinic radiation from any source and of any type can be used in the
process of the invention. The free-radical generating systems activatable
by actinic radiation generally exhibit their maximum sensitivity in the
ultraviolet range, therefore, the radiation source should furnish an
effective amount of this radiation, preferably having a wavelength range
between about 250 nm and 500 nm. In addition to sunlight, suitable
radiation sources include carbon arcs, mercury-vapor arcs, fluorescent
lamps, lasers, electron flash units and photographic flood lamps.
Mercury-vapor lamps, and particularly sun lamps, are most suitable. A
standard radiation source is the Sylvania 350 Blacklight fluorescent lamp
(FR 48T12/350 VL/VHO/180, 115 w) which has a central wavelength of
emission around 354 nm.
The radiation exposure time can vary from fractions of a second to minutes,
depending upon the intensity and spectral energy distribution of the
radiation, its distance from the photohardenable element, and the nature
and amount of the photohardenable composition. The exposure latitude is
the difference between the maximum and minimum exposure times needed to
produce acceptable relief images. The maximum exposure time is usually
determined as the maximum time for which the reverse lines are not filled
in, i.e., fine lines which are intended to be washed out are actually
washed out. The minimum exposure time is usually determined as the time
necessary to hold the highlight dots. In other words, it is the minimum
time needed for the small (2%) dots to remain polymerized and not wash
out. The photohardenable elements made according to the process of the
invention, have a substantially increased exposure latitude and thus
allows the artisan greater flexibility during the plate making process.
The exposure is typically carried out using a mercury vapor arc or a
sunlamp at a distance of about 1.5 to about 60 inches (3.8 to 153 cm) from
the photohardenable element. Exposure temperatures are preferably ambient
or slightly higher, i.e., about 20.degree. to about 35.degree. C.
Following imagewise exposure, the image can be developed by washing with a
suitable developer. Solvent development is usually carried out at room
temperature. Suitable developer solvents for the photohardenable element
prepared according to the process of the invention include aromatic or
aliphatic hydrocarbon or halohydrocarbon solvents. For example, acceptable
solvents include perchloroethylene, 1,1,1-trichloroethane,
tetrachloroethane, trichloroethylene, benzene, toluene, xylene, hexane,
isononylacetate, methylisobutylketone, or mixtures of such solvents with
suitable alcohols. Other organic solvent developers have been disclosed in
published German Application No. 3828551.
Development time can vary, but it is preferably in the range of about 5 to
25 minutes. Developer can be applied in any convenient manner, including
immersion, spraying and brush or roller application. Washout is frequently
carried out in an automatic processing unit which uses solvent and
mechanical brushing action to remove the unexposed portions of the plate,
leaving a relief constituting the exposed image and floor.
Following solvent development, the relief printing plates are generally
blotted or wiped dry, and then dried in a forced air or infrared oven.
Drying times and temperature may vary, however, typically the plate is
dried for about 60 to 120 minutes at 60.degree. C. High temperatures are
not recommended because the support can shrink and this can cause
registration problems. Additional air drying overnight for 16 hours or
more is common. The solvent will continue to evaporate from the printing
relief during drying at ambient conditions.
After the plate has dried, most flexographic printing plates are given an
overall exposure, or post-exposure, using the same radiation source as the
main exposure step. Detackification is also an optional post-development
treatment which can be applied if the surface is still tacky. Tackiness
can be removed by methods well known in the art, such as treatment with
bromine or chlorine solutions as described in Fickes et al., U.S. Pat. No.
4,400,460 or exposure to radiation sources having a wavelength not longer
than 300 nm as described in European Patent Application EP 0 17 927.
The advantageous properties of this invention can be observed by reference
to the following examples which illustrate, but do not limit, the practice
of the invention.
EXAMPLES
In the following examples, all percentages are by weight unless other wise
specified.
The following abbreviations are used:
Binder styrene-isoprene-styrene block copolymer; Kraton.RTM. 1107 from
Shell Chemical Co., Houston, Tex.
PI liquid polyisoprene with a molecular weight of 29,000; LIR-30 Kuraray
Chemical Co., Tokyo, Japan
Piccotex copolymer of vinyl toluene and alpha-methyl styrene; Piccotex.RTM.
100S Hercules Co., Wilmington, Del.
Wax Ceresin wax
HMDA 1,6-hexanediol diacrylate
HMDMA 1,6-hexanediol dimethacrylate
Initiator 2-phenyl-2,2-dimethoxy acetophenone
Inhibitor 2,6-dimethyl-4-t-butyl phenol
HEMA hydroxyethyl methacrylate
Red Dye Neozapon.RTM. Red Dye from BASF Wyandotte Corp., Holland, Mich.
MABS tetrapolymer of methylmethacrylate/acrylonitrile/butadiene/styrene;
Blendex.RTM. 491 from GE Specialty Chemicals, Parkersburg, W. Va.
Blue Dye Atlantic Fast Wool Blue.RTM. Dye from Atlantic Chemicals Co.,
Nutley, N.J.
Example 1
This example illustrates the premixing step of the process of the invention
and the formation of pellets from the premixture.
The following ingredients were charged into a Banbury Mixer and blended at
approximately 150.degree. C.:
______________________________________
Ingredient Amount (parts by weight)
______________________________________
Binder 83.2
PI 9.05
Piccotex 6.64
Wax 1.11
______________________________________
The molten polymer mass from the Banbury Mixer was fed into a single screw
extruder, and then passed through a die plate having holes 3.175 mm in
diameter. The polymer was then chopped into pellets approximately 0.5 cm
long, by a rotating knife on the die plate. The pellets were carried away
by a stream of water containing a small amount of calcium stearate to
prevent agglomeration.
Example 2
This example illustrates the formation of a photohardenable element using
the pelletized premixture from Example 1.
The following ingredients were added to a 92 mm twin screw extruder:
______________________________________
Ingredient Amount (parts by weight)
______________________________________
Pellets from Example 1
87.26
HMDA 5.09
HMDMA 3.57
Initiator 1.94
Inhibitor 1.92
Red Dye 0.0044
HEMA 0.173
______________________________________
The total composition contained 72.6 parts binder; 7.9 parts PI; 5.79 parts
Piccotex; and 0.97 Wax.
The material was then extruded at about 174.degree. C. and calendered
between a support sheet of flame treated polyethylene terephthalate which
was 5 mils (0.013 cm) thick and a multilayer cover element. The multilayer
cover element was prepared by coating a 5 mil (0.013 cm) polyethylene
terephthalate cover sheet with a first layer of Macromelt.RTM. 6900
polyamide (Henkel Corp., Minneapolis, Minn.) at a coating weight of 40
mg/dm.sup.2 ; and a second 1.8 mil (0.0046 cm) thick layer of a blend at a
coating weight of 480 mg/dm.sup.2. The blend contained the following
ingredients:
______________________________________
Ingredient Amount (parts by weight)
______________________________________
Binder 66.7
MABS 31.5
Blue Dye 1.8
______________________________________
The total thickness of the plate, not including the polyethylene
terephthalate cover sheet, was 107 mils (0.272 cm).
Comparative Example 1
In this example, a photohardenable element was prepared without premixing
the binder, plasticizer and aromatic resin.
The same ingredients in the same proportion as in Example 2, but without
premixing, were fed into a 92 mm twin screw extruder and extruded and
calendered as described in Example 2. The total thickness, excluding the
polyethylene terephthalate cover sheet, was 107 mils (0.272 cm).
Example 3
In this example, the exposure latitude was determined from photohardenable
elements prepared according to the procedure described in Example 2 and
Comparative Example 1.
A series of photohardenable elements prepared as described in Example 2 and
Comparative Example 1 were exposed and developed using the same
conditions. Each plate was exposed through the back (flame-treated
polyester support) on a Cyrel.RTM. 30.times.40 exposure unit. Similarly,
the front side of each plate was exposed through a phototool in the same
exposure unit for times varying from 12 to 27 minutes in three minute
steps. The exposed plate samples were washed for 5 minutes in a Cyrel.RTM.
30.times.40 rotary processor using tetrachloro-ethylene/n-butanol (75/25
volume percent) as the developing solvent. The sample plates were then
dried in an oven at 60.degree. C. for two hours. Following drying, the
sample plates were detackified and post-exposed for 8 minutes in a Du Pont
Cyrel.RTM. Light Finish/Post Exposure unit.
For each type of plate, the minimum exposure time to hold a 2% dot (120
line screen), i.e., not wash out the small dots, was determined. For each
type of plate, the maximum exposure time for a 30 mil reverse line to
remain unfilled to a depth of at least 100 micrometers, was also
determined. The exposure latitude was determined as the difference between
the minimum and maximum exposure times. The results given below, wherein
time is measured in minutes, clearly show the superior exposure latitude
of the plates made according to the process of the invention.
______________________________________
Minimum Maximum Exposure
Plate Exposure Exposure Latitude
______________________________________
Example 2 12 25 13
Comparative Ex. 1
12 16 4
______________________________________
The resolution and detail of Example 2 plates and Comparative Example 1
plates were the same when each was properly exposed.
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